This project continues our ongoing work on the effects of various hormones on the growth of muscle cells in culture, and on various processes associated with that growth. The hormones of particular interest are those generally regarded as anabolic agents: growth hormone, insulin, and the somatomedins. Our previous work has established that the somatomedins are the primary stimulators of myoblast proliferation (and possibly also of growth in postmitotic myotubes); consequently our work will concentrate on those hormones of the somatomedin family. In particular, Temin's MSA, which can be isolated in relatively large quantities from the medium in which rat liver cells are grown, serves as the model compound. During the coming year, we will be measuring the binding of MSA to muscle cells and correlating that binding with the rate and extent of its effects on those cells.